A multilayer PCB has multiple copper layers which are interconnected through copper plating vias, such as laser vias, NC vias, buried,blind vias, and plated through vias.
Such a circuit board has copper foil, adhesive, pre-peg, and core materials; all layered together under high pressure and temperature to make a multi-layer PCB. The pressure squeezes out the air, whereas the heat melts the adhesive to bind all layers.
A high-layer count PCB is a board with many layers,from 3 to 16, and more layers. In general, you will see an even number of layers in this reference like 4, 6, 8, 10, 12, 14, 16, 18, 20, etc. A multi-layer circuit board helps engineers create complex products.
A PCB with a high-layer count allowsdesigners to implement high-speed and RF design techniques to have ground returns and a high-power distribution.
The BGA(ball grid arrays) of dense devices demand a multi-layer Printed Circuit Board to teach every possible signal.
Positive Aspects of A Multi-Layer PCB Layout
- A PCB with multiple layers of different materials helps manufacturers create a PCB stack-up that is according to the design specs developed by an engineer.
- A PCB with a single or double layer can’thave a high-speed layout because it can’tcontrol the impedance. It means a multi-layer circuit board is ideal for impedance management.
- You can design a compact PCB with many layers.
- Engineers s can have a better EMI signature by carefully designing a multi-layer PCB, which is not possible with a two-layer circuit board. Whereas the EMC or electromagnetic compatibility also contributes to a product’s development cost, as it becomes less.
Drawbacks of Multi-Layer PCB
- You should add more layers to a circuit board only if it’s important because more layers mean more cost. The fabrication challenges, costs, and the added layers increase the overall cost of a PCB, eventually, the end product becomes expensive. So, follow a pricing strategy and be sure that the extra layers are due to the ROI plan of a product.
- You have to put more time into testing a multi-layer PCB, leading to multiple revisions. So, the production time and the cost may increase.
- While debugging or prototype testing a multi-layer board, you will have to reach every signal on the circuit board.In the case of BGA, QFN, and LGA of a high layer count PCB, it is hard to access or change the signals during the early prototypes.
Types Of A Multilayer PCB
A multi-layer PCB has different types,and all have different layer counts, such as:
Flex PCB: the layer count of this PCB is often 3, 4, and 5 or up to 8
RigidmultilayerPCB: In this case, the layer count consists of 4, 6,8,10, and up to 30.
Rigid-flex:It has from 2 to 20 layers.
Metal core multilayer PCB: It has a maximum of 4 layers.
PCB fabricationof a Multilayer
It is not easy to manufacture a multi-layer PCB because you have to focus on essential details at all stages. You must ensure that all layers are set according to the drilled holes even if there is deformation due tohigh pressure or temperature.
The PCB fabricator must see if the materials are right or not, andthe direction of each layer should be right.It is just like a book where you put different chapters together to make a book. Likewise, every PCB panel is a chapter and 10 chapters are pressed to make a final PCB.Heavy steel plates are used for pressing the PCB panels. The books are then placed in the hydraulic press’s chamber. These presses aredifferent, depending on the material, like if it is polyamide or FR4.
Designing A Stack-Up For A Multi-Layer PCB
The traditional circuit boards have different footprints which are not suitable for multilayer designs. You have to see any additional parameters in this reference and add layers to a footprint, depending on the design software you use. However, many online design platforms offer effective design libraries, so you can take benefit from them.
You should use the latest and precise sources to create the PCB footprints for multi-layer circuits. The planning of a layer set varies in a multilayer board or a double-layer board. We will discuss below some significant points to consider whenplanning the PCB layer stack-up.
Function: You will have to see how the circuitry performs because the final board gets affected by the environment. Like, the properties of its materials can change. You have other latest materials other than just FR4 but they can affect the impedance, changing the design parameters. But, a good manufacturer can help you in this case, due to his experience.
Routing Density:You should also consider the routing density while configuring the layer stack-up. It would causestress if you have to revise the design to add layers after starting the layout, as you have to reconfigure the CAD database and edit the layout. Besides, a circuit board with lots of layers would cost you much more than a board with a few layers.
Overall Cost: The PCB layer count, materials, and configuration directly impact the overall construction cost of a PCB. Your manufacturer knows different options in this respect, so you should discuss them before building a circuit board.
Circuitry Requirements in PCB Design: You can better optimize the layer configuration by considering the requirements of the circuitry. For example, you will have to create a strip line layer configuration in terms of delicate signals to perform well. But, it needs extra ground planes. At the same time, you should separate the digital circuitry and analog having independent ground planes. Moreover, the onboard power supplies should also be isolated.
So, all these steps can affect the layer configuration that should be considered before starting the layout.
The Placement and Routing Of a Multi-Layer PCB
The routing and placement of the board come after you have collected the entire data and finalized the layer stack-up in the database. You have to work in 3D design at certain steps while making the layout for a multi-layer PCB.
In the case of a two-layer PCB, you will have to consider the top and bottom layers. But, differentevents happen internally while working with the multilayer PCB, affecting the top and the bottom of afinal board. Like, you will have to place the noisy component in a specific area to prevent the sensitive routing of an internal layer under it.
The placement of the components involves the same tools as a double-sided PCB, but it will need a different landscape to work with. Like, there is no need to leave more space for creating the routes between components,as they need routing on internal layers. You will have to make short routesdirectly on the surface or top layers to help thesensitive circuitry. However, you have lots of flexibility to work with which is good because a multi-layer PCB involves lots of components.
It is easy to work with power planes and internal routing if you consider certain factors as below:
- You should preplan routing for a multilayer PCB because of lots of components,as they need more routing channels. You might consider specific widths and space for routing, depending on the board technology. Moreover, you must consider the impedance-control traces and differential pairs.
- You should carefully plan the routing of the ground planes, as they have more vias that can affect the return paths of the signals.
- Sometimes routing causes coupling and crosstalk that you can reduce by crossing it in a perpendicular direction on the nearby internal layers. Sometimes routing needs a strip-line structure, so you should root on layers near ground planes.
- You will have to lay down the strip planes to prevent the delicate signals from crossing the splits and damaging the return path. Such an issue can cause noises on the multilayer circuit board.
Making Drawings and Files
After component placement and routing are over and tested, you can follow the same design process as that of a double-sided PCB. The board is now ready to fabricate,but you have to make the documents for manufacturing. The stack-up and the PCB construction details should be made.In the case of Gerber files, a multiple-layer PCB involves more manufacturing output files than a normal board.
The DFM for A Multi-Layer PCB
The design for manufacturability involves the following factors:
- Considering via clearance in terms of internal layers.
- Selecting a manufacturer according to his capabilities.
- The copper of internal layers should be 10mil away from the board’s outer edge, however, 20mil would be better.
- In the case of thermal relief pads, the tie has to be at least 8mil or more is better.
- Larger geometries involve more cost, as they cause high yields.
The construction of a multi-layer PCB is different from a board with one or two layers. Such a circuit board has copper foil, adhesive, pre-peg, and core materials like aluminum, FR4, ceramic, etc. A PCB with a high-layer count allows designers to implement high-speed and RF design techniques to have ground returns and a high-power distribution.
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